Pinout

AMS1117 linear regulator (SOT-223) pinout

Pins

1: GND on a fixed-voltage part (3.3V, 5V, …) — ADJ on the adjustable version, tied to its own feedback divider
2: VOUT — electrically the SAME node as the metal tab
3: VIN
Tab: VOUT, not ground — do not bolt it to a grounded heatsink without insulation

Output voltage	Fixed 1.5, 1.8, 2.5, 2.85, 3.3 or 5.0 V, or adjustable (1.25 V reference)
Output current	1 A
Dropout	≈ 1.1 V typ, 1.3 V max at 1 A
Input voltage	Vout + 1.3 V minimum; 15 V absolute max
Quiescent current	≈ 5 mA typ (fixed versions)
Output capacitor	≥ 22 µF tantalum per the datasheet (needed for stability)
Protection	Current limit and thermal shutdown
Tab	Connected to VOUT, not ground
Package	SOT-223 (also TO-252)

The regulator on the back of an enormous share of 3.3V breakout boards and Arduino clones — if a board has an unlabelled 3-lead SMD part with a wide metal back next to its power jack, this is the usual first guess.
Needs input/output capacitors (typically 10µF/22µF per the datasheet's own reference circuit) right at its pins to stay stable — an AMS1117 with no capacitors nearby is a common cause of a "regulator" that oscillates or resets under load.